In the quest for improved performance, electronic circuits are becoming denser and the devices therein are becoming smaller. For example, the most common dielectric in metal oxide field effect transistors (MOSFETs) has been SiO2. However as the thickness of SiO2 approaches 15 Å, substantial problems appear, including, for example, leakage currents through the gate dielectric, concerns about the long-term dielectric reliability, and the difficulty in manufacturing and thickness control.
One solution to the above problem is to use thick (greater than 20 Å) films of materials, such as hafnium oxide (HfO2), that have a dielectric constant that is larger than SiO2. Thus, the physical thickness of the gate dielectric can be large, while the electrical equivalent thickness relative to SiO2 films can be scaled.
Introduction of high-k dielectrics, such as HfO2, Zro2 or Al2O3, in gate stacks has proven to reduce leakage current by several orders of magnitude. Such leakage current reduction has enabled the fabrication of complementary metal oxide semiconductor (CMOS) devices with lower power consumption. Unfortunately, other problems have arisen from utilizing high-k dielectrics in CMOS devices including difficulty of passivating the underlying silicon, the introduction of unwanted charges in the gate stack that produce large flat band voltage shifts, large threshold voltage shifts, significant charge trapping and low electron mobility devices.
Indeed, it has been reported that the electron mobilities of metal gate electrode/high-k gate dielectric stacks formed on a silicon substrate are severely degraded when compared with conventional poysilicon/SiO2 gate stacks. See, for example, Callegari, et al., Int. Conf. SSDM, September 16–18, Tokyo, Japan 2003. Despite having degraded electron mobilities, the use of high-k gate dielectrics in the next generation of very large scale integrated (VLSI) circuits is necessary to reduce leakage currents in CMOS devices. Remote phonon scattering or remote charge scattering have been suggested to explain mobility degradation for nFETs. See M. V. Fischetti, et al., “Effective Electron Mobility in Si Inversion Layers in MOS systems with a High-k Insulator: The Role of Remote Phonon Scattering”, J. Appl. Phys. 90, 4587 (2001) and M. Hiratani, et al. JJAP Vol. 41, p. 4521 (2002).
In high-k dielectrics, such as HfO2, a metal-oxygen bond is easily polarizable under an external electric field, which results in highly undesirable scattering of channel mobile charges by remote phonons present in the high-k material. As the result, the MOS device drive current can be substantially reduced by the presence of high-k materials as the gate insulator. Several existing solutions are directed to the reduction of the scattering problem. In one known solution, a layer of silicon oxide or silicon oxynitride is disposed between the channel located within the Si substrate and the high-k gate dielectric. Some of the remote phonon scattering is reduced using these so-called interlayers because the high-k gate dielectric is positioned further away from the channel.
Although prior art gate stack structures (including a conventional interlayer and high-k dielectric) have reduced remote phonon scattering, they still do not achieve the electron mobility of MOS devices that contain SiO2 as the gate dielectric. Hence, there is still a need for providing a MOS device stack, which contains a high-k gate dielectric and a metal gate, that has improved electron mobility that is substantially equivalent to conventional SiO2-containing MOS devices.